Improve error message when trying to access a completed delivery link

When we send out a delivery link, the recipient may forward that along, thinking it will allow others to access the data.

However, when another user attempts to view the delivery (and logs in), they see the following error (internally from Data Service):

> Delivery Service Error
>
> Reason: Error 403 on /project_transfers/xxxxxx
> Reason:Unauthorized Suggestion:request permission to access this resource

1. The error message should be more informative - describing why the user's action failed.
2. It's also a good idea to update the email text to inform recipients that others won't be able to access their deliveries

Trace the delivery-link access flow around the /project_transfers/xxxxxx authorization error, then locate the email text sent with delivery links. Confirm how completed deliveries are handled for a different logged-in user and identify the existing tests or checks for these messages. Done means the access failure explains the cause and the email warns that forwarded deliveries cannot be accessed by others.

Written by the indexing model from the issue text.